Slice onions into rings (about 1/8 thick). Heat 2 tbsp oil in 12″ skillet, add onions and cook on moderately high heat, but do not brown, just sweat them. After onions begin to soften, add cider vinegar and brown sugar. Stir frequently, and cook for about 20 minutes. Onions should gradually darken. Total cooking time can be as little as 25 minutes, not the hours you read about for caramelized onions! You can put the cooked onions in a plastic baggie to freeze, and just chop off the amount you need for a recipe.